1. Field of the Invention
The present invention relates to a moment-resistant structure, sustainer, and method of construction for deformably resisting episodic loads, particularly those of high intensity. The episodic loads may be due to earthquake, impact, or other intense episodic sources. The structure and sustainer may be in buildings, bridges, or other civil works, land vehicles, watercraft, aircraft, spacecraft, machinery, or other structural systems or apparati. The sustainer is a rigid member which resists transverse loading and supports or retains other components of a construction, such as a joist, a beam, a girder, a column, or any member which resists transverse loading. The structure or sustainer may be comprised of metals, such as steel, iron, aluminum, copper, or bronze, or of wood or wood products, or of concrete, plastics, other polymers, fiberglass or carbon fiber composites, ceramics, or other materials or combinations involving these and other materials.
2. Description of Prior Art
Steel structures generally had been regarded by structural engineers and architects as providing excellent resistance to earthquake motions, in large part owing to the substantial deformation capacity of steel members observed in laboratory and field studies. However, the 1994 Northridge earthquake caused unexpected, severe, and widespread damage to steel moment-resistant frame structures in the Los Angeles area. Much of the damage to steel moment-resistant frames occurred at or near the welded connections between steel girders and columns. In some buildings over 80 percent of the connections were found to have had brittle fractures at the connection welds or in girder or column material adjacent to the welds. Concern was such that numerous experimental and analytical research studies were initiated to determine the cause of the fractures and to determine applicable solutions for the design of new steel structures and for the rehabilitation of existing steel structures.
The Japanese also had believed steel structures had superior resistance to earthquakes, but brittle failures at or near connections like those observed in Los Angeles were found after the 1995 earthquake that shook Kobe. Fractured beam-column connections were also observed in recent inspections of steel buildings in the San Francisco Bay Area, possibly resulting from the 1989 Loma Prieta earthquake.
The causes of these fractures are attributed to the following possible sources: the welding procedure and conditions, the use of backup bars and run-off tabs, the characteristics of the girder and column material, and configurations that cause triaxial restraint to develop in the vicinity of the welds. The fractures occurred more often at or near the bottom flange weld, and this is believed to result from difficulties in achieving acceptable welds because physical access to the bottom flange is impeded, and because the floor above the beam protects the top flange and forces the bottom flange to experience larger strength and deformation demands. With regard to material characteristics, attention focuses on the fracture toughness of the materials, weld material deposition rates, and through-the-thickness variations in material properties of the column flanges. In addition to these potential causes, stress and strain concentrations naturally arise at junctures, such as at a girder-column connection. Due to the above variables, it can be seen that the strength of a girder-column connection cannot be predicted with certainty and can only be estimated.
Research into the causes of the fractures and possible solutions is ongoing. Laboratory tests of full-size specimens have fractured at small deformations, reproducing the behavior apparent in the field. Techniques for the repair of fractured connections, for the rehabilitation of existing, undamaged connections, and for the design of new structures have been tested. Even the best of these have limited deformability, are costly, and may be unreliable.
The approaches and solutions investigated to date concern (1) achieving improved material deformability characteristics through controls on welding materials and procedures, (2) relieving conditions of triaxial restraint by "softening" the region near the welds by removing some girder and/or column material, thus lessening the degree of restraint, (3) providing new details for ductile connections, designed with the intention that inelastic deformations should take place within the connection rather than in the girder, (4) weakening the girder flanges in specific locations so that inelastic flexural deformation of the girder takes place in zones located at some distance from the girder-column connection, (5) strengthening the connection to shift inelastic flexural demands to the girder, away from the column face, and (6) combinations of the preceding. For some of these approaches ((3), (4), and (5)), the connection is protected from inelasticity by providing weaker elements that will deform or plastify at lower loads.
A basic tenet in earthquake-resistant structural design is that savings in structural weight and cost can be obtained if the structure is designed and detailed to respond in a ductile, inelastic fashion. A second basic tenet in earthquake-resistant structural design is that ductile, inelastic response should preferably take place in plastic hinge zones located in the beams and girders of a frame rather than in the columns. The reason for this second tenet is concern that the integrity of a column may be compromised if it developed a plastic hinge, and this could jeopardize the stability of the numerous floors that may be supported above. Existing design practice provided for the formation of plastic hinge zones in the beams and girders, adjacent to the columns, and consistent with these tenets.
Steel moment frames were used frequently in earthquake-prone areas, due to market forces and the mistaken belief that this structural system had ample deformation capacity. Perhaps because of this belief some inherent disadvantages of the system were overlooked or tolerated. Note that:
Frames subjected to seismic loading experience the largest stress and strain demands in their most vulnerable locations--at the beam-column connection where the connection welds and heat-affected zones are located. PA1 The steel provided to the construction may have varied strengths relative to the strengths assumed in the design. Where the strength of the girders is relatively high, an increased likelihood results that plastic hinges develop in the columns. PA1 The presence of a floor slab supported by an underlying girder can increase the flexural strength of the composite slab-girder. This unanticipated strength may have the undesirable effect of forcing plastic hinges to develop in the columns. PA1 The concentration of inelasticity into relatively small locations (plastic hinges) requires the material to undergo very large strain demands locally. Distributing the inelastic demands over larger volumes of material would reduce the local demands, and enhance the displacement capacity of the structure. PA1 The conventional practice of using unperforated beams and girders requires that additional space be provided for service utilities between the ceiling and the structural framing. PA1 The conventional practice makes no provision for the post-earthquake restoration of the structure. Repairs may be so costly as to warrant replacement of the building, or cumbersome rehabilitation. PA1 (a) the provision of dissipative zones capable of absorbing or dissipating substantial amounts of distortional vibration energy; PA1 (b) the provision of dissipative zones capable of sustaining large deformation demands distributed over the length of the girder web; PA1 (c) the provision of dissipative zones that are subjected to predominantly biaxial or plane stress conditions, thus preventing conditions of triaxial restraint such as occur at conventional beam-column connections that limit the ductility and strain capacity of the material; PA1 (d) the advantageous use of strain hardening properties of the material to cause the spread of inelasticity to multiple dissipative zones, thus offsetting the tendency for strain concentrations to develop because of deviations from ideal conditions owing to material, workmanship, and loading variations, thereby achieving a robust system for providing deformation capacity; PA1 (e) the efficient use of structural material, because deformation demands are distributed to numerous dissipative zones located over the member length, avoiding the concentration of deformation demands in localized areas and the potential for material exhaustion in these areas; PA1 (f) the provision of a structural fuse, that by yielding of the web, regulates the forces and bending moments resisted at the beam-column connection, thereby protecting the beam-column connection from stress and strain demands that, if excessive, i.e., if exceeding the beam-column connection's strength capacity, would likely cause brittle fracture of the welds or adjacent beam or column material; PA1 (g) the requirement that welds only be of sufficient quality to prevent fracture of the welds or adjacent beam and column material for the reduced forces and bending moments associated with the deforming dissipative zones, thereby avoiding the demands and expense of current practices; PA1 (h) the achievement of a connection of sufficient strength to force inelastic demands to occur in the girder away from the connection by regulating the forces and bending moments resisted at the beam-column connection without the expense of current practices; PA1 (i) the limitation of stress and strain demands, that if excessive, might cause brittle failure of the column flange because of the inferior material properties of relatively thick column flanges by regulating the forces and bending moments resisted at the beam-column connection; PA1 (j) the reduced possibility that the strength of the girder might exceed the strength of the column, by regulating the forces and bending moments resisted at the beam-column connection, thereby helping to prevent plastic hinges from developing in the column; PA1 (k) the reduced possibility that contributions of the floor slab to the flexural strength of the girder can force inelasticity to develop in the columns because the shear force that may be carried by the girder is regulated; PA1 (l) the reduced possibility that variability in materials strengths leads to uncertainties in the mode or locations of inelastic response by utilizing girders composed of the same material throughout, thus causing the shear strength of the girder to vary in proportion to the flexural strength of the connection; PA1 (m) the reduction in complications arising from the three-dimensional configuration and interaction of beams, girders, and columns by regulating the strength of the beams and girders; PA1 (n) the achievement of flexibility in floor space usage by not requiring the use of diagonal members; PA1 (o) the reduction in materials requirements and cost achieved by providing apertures in the webs of the beams through which mechanical equipment and utilities may pass, thereby allowing reduced story heights and allowing more floors to be built in regions with zoning restrictions on building height; PA1 (p) the expeditious and economical restoration of the lateral force-resisting qualities of a structure by providing for the replacement of girders after a damaging earthquake; PA1 (q) the economy with which the web openings can be fabricated relative to the expense required to cut the flanges or provide other means for improving the displacement capacity of the structural system; PA1 (r) the economy with which the web openings can be introduced into existing structures compared with the effort and expense required to implement other retrofit techniques; PA1 (s) the ease with which the structural system can be modeled for purposes of determining design forces and displacements relative to other structural systems; PA1 (t) the ease with which the structural system can be designed relative to other systems because the one or more voids have slight or negligible effects on the stiffness of the structural system; and, PA1 (u) the latitude given to the structural engineer to reliably specify locations where inelastic response may develop and modes of inelastic response, thereby giving the engineer the ability to control the displacement capacity and response characteristics of the structure.
Attempts to remedy the fracture problem have consistently embraced the flexural yielding paradigm despite the disadvantages noted above.
Improving the quality of the welds and base materials, or increasing the connection strength adequately to promote the development of plastic hinges in the beam away from the connection is expensive.
Details required to relieve triaxial restraint are also costly. Experimental evidence indicates that these techniques provide only moderate levels of ductility capacity; peak stresses continue to occur at the beam-column connection, and weld quality remains extremely important to the ductility capacity of the connection.
Other connection details have been proposed to protect the connection from overstress by promoting yielding in the body of the connection rather than in the girders or columns. These connections are costly to implement in the field, and affect the stiffness of the building, which in turn affects the required lateral design strength and its displacement response and deformability demand. Often it is not possible to configure these connections to support beams and girders framing into various sides of a column simultaneously.
The girder may be intentionally weakened by reducing the flange cross section to promote plastic hinging at a location offset from the connection to the column, representing a worthwhile attempt to draw inelastic action away from the welded beam-column connection where brittle failures might initiate. But this approach has its disadvantages: (1) it is relatively costly to cut the flange at four locations at each end of the beam; (2) it is not practical to cut the top flanges where floor slabs may be present in the rehabilitation of existing construction; (3) because the plastic hinge zones are set in from the columns, they are subjected to larger deformations to achieve the same displacement of the structure; (4) heavier, more costly beams must be used in order that the cross section having reduced moment capacity provide the system with adequate strength; (5) the removal of flange material reduces the stability of the beam, thereby limiting its deformation capacity; and (6) the asymmetrical removal of flange material, as may happen recognizing the inexactness with which the flange cuts may be executed, may induce instabilities, further limiting the deformation capacity.
While the foregoing approaches concern recent suggestions to improve steel moment resistant frames, other approaches to earthquake resistant design merit some discussion and bear on the invention.
The eccentric-braced steel frame was developed by Popov in the 1970s and 1980s. In this system, diagonal braces are offset from the beam-column connections in order to develop an eccentricity between the brace and the beam-column working point. This induces high shears on a short segment of the beam, causing it to yield principally in shear under strong lateral motion. The shear yielding of this link beam is the only intended zone and mode of inelastic response. The large shear strains that the link beam is capable of sustaining provides the inelastic deformability of the system. The eccentric-braced frame has been used in a number of structures, some which were shaken by the Northridge earthquake and reportedly performed quite well. Widespread adoption of the system has been limited by its higher cost and the presence of the diagonal brace, which interferes with floor space utilization. The cost of this system is bound to increase as it becomes necessary to provide more control over the quality of the welds. As for flexural yielding systems, the eccentric braced frame imposes relatively high local strain demands because the zones of inelasticity are relatively few in number and small in size.
Alternative approaches to earthquake resistant construction are also being developed. Of particular interest are the use of supplemental damping devices. One such device, the ADAS element, is configured with an hourglass shape so that yielding in flexure develops inelastic response throughout the volume of the material rather than in discrete zones near the member ends. Another device causes steel plates to yield in shear. Nakashima reports very desirable properties for a steel used in this manner for purposes of controlling response to earthquakes, including stable, ductile hysteretic response to large strains over a large number of loading cycles. This device would be positioned between an oscillating structure and a rigid frame. Another approach incorporates a lead plug in the center of a base-isolation bearing to provide additional stiffness and damping. These three methods all show good performance in the laboratory, but significant cost and architectural accommodations are required to providing the support systems required to use these devices. They also require specialized knowledge and analysis to implement. These aspects hinder their use in mainstream construction.
After a damaging earthquake it is usually necessary to evaluate the integrity of the structural system, to determine whether it is able to resist future earthquakes, or whether repairs or more extensive rehabilitation is needed. The judgement of the engineer is often relied upon, because existing standards are not broad enough in scope and because it is not possible to accurately determine the loss in capacity, if any. Options are limited, because conventional structural systems are not designed for the replacement of damaged elements. It is generally easier to replace supplemental damping devices in alternative structural systems, but other aspects hinder their broad acceptance.